FISHERIES OF THE UNITED STATES. [24] 



Fig. 28 thus becomes converted iuto the walls of the digestive tract, 

 and tlie space which is outside aud below the embryo iu Fig. 1*8 becomes 

 converted into an inclosed digestive cavity, which opens externally by 

 the primitive mouth. 



"This stage of development, in which the embryo consists of two 

 layers, an inner layer surrounding a cavity which opens externally by 

 a mouth-like opening, and an outer layer, which is continuous with the 

 inner around the margins of the opening, is of very frequent occurrence, 

 and it has been found, with modifications, in the most widely separated 

 groups of animals, such as the star-fish, the oyster, and the frog, and 

 some representatives of all the larger groui)s of animals, except the 

 Protozoa, appear to pass during their development through a form 

 which may be regarded as a more or less considerable modification of 

 that i^resented by our oyster embryo. This stage of development is 

 known as the gastrula stage. 



"Certain full-grown animals, such as the fresh-water hydra and some 

 sponges, are little more than modified gastrulas. The body is a simple 

 vase with an opening at one end communicating with a digestive cav- 

 ity, the Avail of which is formed by a layer of cells which is continuous 

 around the opening with a second layer which forms the outer wall of 

 the body. This fact, together with the fact that animals of the most 

 widely separated groups pass through a gastrula stage o(:'develoi)ment, 

 has led certain naturalists to a generalization, which is known as the 

 'gastrula theory.' This theory or hypothesis is that all animals, ex- 

 cept the Protozoa, are more or less direct descendants of one common 

 but very remote ancestral form, whose body consisted of a simple two- 

 walled vase, with a central digestive cavity opening externally at one 

 end of the body. 



"Hieckel, who is the originator and leading advocate of this hypothe- 

 sis, has proposed to call this ancestral form a 'Gastra^a;' and the gas- 

 trula stage of development he regards as a trace or indication of this 

 distant ancestry, which is still retained and passed through during the 

 early stages of the development of animals which are now very widely 

 separated. 



"The gastrula theory cannot be regarded as one of the established 

 generalizations of science, and the evidence which has so far been ac- 

 cumulated by embryologists is not by any means straightlbrward or 

 satisfactory. The theory is one of the most interesting end)ryological 

 problems under discussion, however, and any new information which 

 bears upon it is of value. 



"The fact that the oyster goes through a very well marked and very 

 slightly modified gastrula stage is therefore of great theoretical inter- 

 est, and more so since Salensky, a distinguished liussian embryologist, 

 has proposed in place of the gastrula theory another theory, which is 

 based, in ])art, upon erroiu^ous observations upon the development of 

 the oj'ster, which Salensky says does not pass through the gastrula 



